T helper (Th) cells play a pivotal role in the immune response primarily by directing responses of other lymphoid cells. They induce activation and generation of effector function and can modulate (enhance or suppress) an ongoing response. Regulation can occur either through cell- cell contact (e.g. activation of B cells) or through secretion of cytokines which have varied effects on multiple cell types within the immune system. The type of Th response elicited can also be affected in much the same way, for example by cell presenting Ag to the Th (B cells vs. macrophage vs. dendritic cell) or by additional cytokines produced during the early non-specific phase of the immune response (e.g. NK secretion of IFN-gamma). Knowledge of the fundamental processes that govern an immune response especially from its initiation through the primary response, where much of the regulation and preprogramming is likely to occur, up to the development of memory, are of utmost value in understanding how a particular immune reaction is generated to different pathogens, and how adverse responses may occur. More significantly, with this information strategies can be formulated for enhancing protective responses and neutralizing or downregulating potentially damaging situations. This proposal will examine events that take place and shape the primary immune response including which Ag-presenting cells can initiate the naive Th response and which are involved in perpetuating the response, which factors influence the cytokine secretion capacity of Th and the type of cytokines produced, and what are the requirements for promoting Th driven B cell responses and humoral immunity. Experiments and what are the requirements for promoting Th driven B cell responses and humoral immunity. Experiments will utilize Th from TCR transgenic mice which are in a naive state, having never encountered Ag, and are specific for a defined peptide of Pigeon Cytochrome C. We will study the influence of varying types of APC and the role of APC costimulatory molecules in initiating the naive cell response, and in generating effector Th capable of secreting different cytokines and eliciting different functions in the primary response. In particular, we will study the effect of repeated antigenic stimulation on cytokine secretion, preliminary results indicating that contrasting Th1 or Th2 phenotypes can be generated depending on the frequency of stimulation, and the type of cell presenting Ag. Also, we will study factors influencing the ability of naive Th to help B cells, concentrating on the induction and expression of CD40-L on Th and its role in B cell activation. From the information we will gather in vitro we will then develop an in vivo model in which we will determine the exact sequence of events which take place during primary responses in lymphoid organs. We will transfer naive Th into adoptive hosts, stimulate with Ag in vivo, and then visualize the course of the primary response using immunohistology. We will assess the sites of naive activation and the APC involved in directing events, the migration and localization of Th over time, the interaction of Th and B cells and the formation of lymphoid follicles, and the sites and nature of cytokine secretion in situ. Altogether these studies should provide a very comprehensive picture of the events occurring during the primary Th response and highlight ways to regulate or alter the nature of an immune response.